1. Field of the Invention
The present invention relates to an in-cylinder injection type internal combustion engine adapted to inject fuel directly into a combustion chamber, and particularly to such an in-cylinder injection type internal combustion engine that is suitably constructed to reduce discharge of unburned HC immediately after a cold start of the engine.
2. Discussion of Related Art
Recently, in-cylinder injection type internal combustion engines adapted to inject fuel directly into combustion chambers have come into practice. In such an in-cylinder injection type engine, the timing of fuel injection can be set as desired, allowing the engine to operate in an extreme lean-burn mode, utilizing a so-called stratified charge combustion, while the engine is in a low loaded operating region. In this lean-burn mode, the fuel is injected during a compression stroke so that an air-fuel mixture, whose fuel concentration is large enough to cause firing, is concentrated in the vicinity of a spark plug, to thus enable the stratified charge combustion.
When the above type of in-cylinder injection type engine starts operating in a cold state, or makes a cold start, it is desired, as in conventional engines, to activate a catalyst(s) provided in an exhaust passage, in the early stage of engine operation, to reduce the amount of unburned HC and other harmful substances that are released to the atmosphere.
In view of the above situation, it has been proposed in Japanese laid-open Patent Publication No. 8-100638 relating to in-cylinder injection type engines, to perform an additional fuel injection during the initial period to middle period of the expansion stroke, independently of fuel injection for main combustion, and ignite the additional fuel injected by the fuel injection, utilizing flame propagation from the main combustion, to increase the exhaust gas temperature, and accelerate warm-up of the catalyst for early activation of the catalyst.
Where the additional fuel injection is performed separately from the fuel injection for main combustion as described above, combustion of the injected additional fuel takes place over a region ranging from the inside of the combustion chamber to the inside of the exhaust port. If all of the additional fuel is completely burned, the resulting exhaust gas will contain almost no unburned HC.
In the meantime, the exhaust passage, through which the exhaust gas is discharged from the combustion chamber into the exhaust port, has a considerably small cross sectional area at the moment that an exhaust valve is opened. Upon opening of the exhaust valve, a high-pressure exhaust gas rushes out of the combustion chamber, into the exhaust port, and the flow rate of the exhaust gas is considerably high in the initial period of opening of the exhaust valve (where the crank angle is in the range of 135.degree. to 180.degree.). The exhaust gas having such a high flow rate in the initial period of opening of the exhaust valve is called "blow-down gas." Consequently, the additional fuel that was burning in the combustion chamber has its flame extinguished while it is flowing at a high speed with the blow-down gas, through the narrow exhaust passage just after opening of the exhaust valve, and part of the additional fuel that is left unburned is discharged as unburned HC, along with the blow-down gas.
The exhaust gas flowing from the combustion chamber into the exhaust port is discharged into an exhaust manifold to which a plurality of exhaust ports are connected, and mixed in the exhaust manifold with exhaust gases discharged from other cylinders, to be fed to the catalyst(s) located downstream of the exhaust manifold. At this time, even if an exhaust gas discharged from a certain cylinder contains unburned HC, the exhaust gas may be mixed with exhaust gases from other cylinders that are still burning, and the unburned HC can be expected to be re-burned.
In the known in-cylinder injection type internal combustion engine, however, the exhaust manifold consists of a pipe-connection type manifold 50 (used for a four-cylinder engine in this example), in which a plurality of pipes 55, 56, 57, 58 are connected together, as shown in FIGS. 17(a) and 17(b). To prevent reduction in the output due to interference of exhaust gases, the cylinders in which non-continuous combustion occurs are connected to each other through pipes, and the pipes 55, 56, 57, 58 are formed with minimum amount of curves so that exhaust gases can smoothly flow out of the pipes.
In the exhaust manifold 50 as described above, blow-down gas discharged at a high flow rate from each of the exhaust ports 51, 52, 53, 54 flows through a corresponding one of the pipes 55, 56, 57, 58, without being mixed with blow-down gases discharged from the other exhaust ports. Since the pipes 55, 56, 57, 58 that extend from the respective exhaust ports 51, 52, 53, 54 to a joining portion 59 are formed with a relatively large pipe length, combustion gases that were burning when discharged from the exhaust ports 51, 52, 53, 54 are cooled down by the time when the gases reach the joining portion 59, with a high possibility that the temperature of the gases is lower than the temperature that permits reaction between unburned HC and the combustion gases.
Accordingly, the exhaust gas containing unburned HC is unlikely to be mixed with exhaust gas that is still burning, for re-combustion of the unburned HC.
FIG. 18 shows changes in the pressure within the combustion chamber and changes in concentration of unburned HC, along with an ignition signal and a fuel injection signal, in the in-cylender injection type internal,combustion engine including the exhaust manifold as shown in FIGS. 17(a) and 17(b). Among the curves that represent changes in the concentration of unburned HC, the solid line indicates the concentration of unburned HC at point "a" in the exhaust port 51, and the broken line indicates the concentration of unburned HC at point "b" in the joining portion 59.
As shown in FIG. 18, unburned HC that is left unburned in the exhaust port 51, for example, is only slightly oxidized in a high-temperature atmosphere of exhaust gas, and its amount is reduced only by a small degree. Namely, the unburned HC from the exhaust port 51 is discharged from the exhaust manifold 50 into a downstream passage, while maintaining its high concentration, without being re-burned in the joining portion 59 while being mixed with blow-down gases discharged from the other exhaust ports 52, 53, 54.
FIG. 19 shows the temperature at the center of a catalyst during a cold start operation of the engine, and the HC concentration measured at the outlet of an exhaust manifold, with respect to the known in-cylinder injection type engine having the pipe-connection type manifold, and another known example of internal combustion engine (referred to as MPI (multi-point injection) engine in FIG. 19).
As shown in FIG. 19, when additional fuel injection is performed in the in-cylinder injection type engine as disclosed in the above-identified publication, the time required for the catalyst to be activated with its center temperature increased can be significantly reduced as compared with the known MPI engine. However, there is still a problem that unburned HC is released to the atmosphere until the catalyst is activated. Furthermore, unburned HC is produced by the additional fuel injection, in addition to that produced by the main combustion, and therefore the amount of unburned HC that remains until the catalyst is activated is larger than that of the known MPI engine.
In particular, if the additional fuel injection is conducted over a period from the initial period to middle period of the expansion stroke as disclosed in the above-identified publication, a large amount of unburned HC is produced, and a part of the energy of the additional fuel is used for increasing the pressure within the cylinder, resulting in variations in the output torque due to the increased pressure within the cylinder. Also, the temperature of the exhaust gas is not so largely increased, and therefore the activation of the catalyst does not proceed at a high speed.